1. Field of the Invention
The present invention relates to an electroless gold plating solution, and more particularly, to an electroless gold plating solutions which will not damage ceramics and has a high deposition rate with high stability.
2. Description of the Related Art
Electroless plating is a very important means of plating a portion to which an electrical conduction cannot be obtained. The electroless solutions actually used for this process must be stable during use.
Although many various electroless plating solutions are in actual use, a usable electroless gold plating solution has not heretofore been developed. This is due to the very low stability of such electroless gold plating solutions.
The generally used electroless gold plating solutions contain alkaline hydroxide, alkali metal cyanide, boron hydrides, amino boranes and a water-soluble gold salt additive which acts as a metal supply source. However, such electroless gold plating solutions will be decomposed even if a small amount of nickel (few ppm) is dissolved in that solution.
For example, a ceramic package for a semiconductor device, is produced by forming a metallized conductor pattern on a ceramic substrate, forming a nickel electro- or electroless plating coating on the metallized conductor layer, and forming a gold plating coating on the nickel coating.
If the gold plating coating is formed by the above-mentioned electroless gold plating process, the nickel of the underlayer (substrate) will be dissolved in the solution and the solution is decomposed. Thus, the nickel substrate is first coated with a thin layer of immersion deposited gold before it is placed in real electroless (auto catalytic) gold plating solution. But the gold plating coating obtained by the immersion is extremely thin and porous so that the nickel of the under layer is dissolved during the electroless gold plating process. This dissolved nickel leads to less selective plating of gold and gold is deposited on the ceramic and finally excess nickel leads to spontaneous decomposition of the solution.
Due to the above problems associated with known solutions, it has been almost impossible to carry out an electroless gold plating of the nickel underlayer. In the case of the above-mentioned ceramic package, an electroplating process will be done after isolated patterns are all short-circuited. In this case, the connected conductor pattern must be removed after the gold plating.
Another problem is high alkalinity of commercially available electroless gold plating solutions. Such a strong alkaline gold plating solution corrodes ceramics, whereby the surface of the ceramic substrate is coarsened. This surface roughness lowers the quality of products. Further, since the silica, etc., of the ceramic components is dissolved in the plating solution, the life of the expensive gold plating solution is shortened.
To obtain an electroless gold plating solution which does not corrode ceramics, the pH of the plating solution must be lowered to less than 13, but when the pH of the gold plating solution is lowered to such a level, the gold deposition rate is lowered to 0.2 .mu.m/hour or less.